Secure deletion of data on non-volatile magnetic data storage, such as a hard disk, is an act of securely purging the data such that there are no residuals of the data on the hard disk. Regulations exist which mandate the need for secure deletion according to various standards, such as that published by the U.S. Department of Defense. A hard disk drive includes one or more hard disks clamped to a rotating spindle and at least one head for reading or writing data on the disk. Storage on a hard disk is divided into “blocks”, which are in turn grouped into “tracks”. A disk head uses magnetism to read or write data onto the tracks. Secure deletion involves the overwriting of blocks on a track by the disk head numerous times, depending on the standard being applied. Thus, secure deletion involves multiple input/output (I/O) operations on the same blocks of a track.
As a track on the hard disk is written, adjacent tracks may be overwritten by the magnetic field generated in the disk head, and data recorded in the adjacent tracks may be erased or corrupted as a result. This phenomenon is called adjacent track erasure (ATE) or adjacent track interference (ATI) and are aggravated by the multiple overwrites required by secure deletion.
In one approach to counter ATE or ATI, when a track has been overwritten a certain number of times, the adjacent tracks are read and rewritten, hence refreshing the data on the adjacent tracks. However, the I/O operations required in the refreshing of the adjacent tracks impose a performance penalty on the hard disk drive. With the aggravation of the ATE and ATI phenomenon due to secure deletion requirements, the need to refresh adjacent tracks become more frequent, imposing an even high performance penalty on the hard disk drive.